The brain is a complex organ containing hundreds of different cell types that originate from neural stem cells, which initially all look alike. How is such enormous cell diversity achieved? Stem cells have to make important decisions such as to proliferate or to stop proliferating, to die or to stay alive and to become a certain cell type rather than another. Proper coordination and control of such decisions have a direct influence on the generation of the myriad of cell types that ultimately make up a fully functional brain. The major research interest our lab is to uncover the cellular and molecular mechanisms that underlie this cell diversification process. The understanding of these mechanisms is a critical first step towards using neural stem cells as a source of new cells to replace those lost after neural injury or neurodegenerative diseases such as, for example, Parkinson, Alzheimer and various retinopathies.
It is believed that both cell-intrinsic mechanisms (gene expression) and extracellular signals play a part in controlling how cell-fate decisions are made, but their relative contributions remain unclear. As progenitor cells of the retina are multipotent and can generate seven major cell types, which can all be identified by expression of cell-type specific markers, we use the vertebrate retina as a model system to address the problem of cell-fate determination. We recently provided evidence that cell-intrinsic mechanisms play a major part in controlling cell-fate decisions in the retina, but how such intrinsic mechanisms work to specify cell fate remains unknown. Current projects in the lab are aimed at uncovering these intrinsic developmental programs. Specifically, we are interested in addressing the following questions.
1) Do intrinsic mechanisms depend on pre-determined intracellular programs or do they depend on stochastic choices?
We are using a clonal-density culture system and a retinal explant assay to address this question. By watching the development of individual retinal progenitor cells (RPCs) using live imaging microscopy, we can reconstruct the entire lineage of a progenitor cell (its lineage "tree"). Using this approach, we hope to determine whether strict rules operate within a lineage to control when a particular cell type is generated or whether various cell types are generated randomly throughout the lienage.
2) How do intrinsic signals operate in RPCs to help generate different cell types at different times?
It is known that RPCs change as development proceeds, gaining and loosing the competence to generate particular neuronal and glial cell types, but the molecular nature of these changes in competence remains unknown. Projects are underway to identify key genes that control how RPCs become different from one another and how they change over time to generate different cell types at differnt times. To study the function of these genes in developing RPCs, we use retroviral vectors to manipulate their expression in gain-of-function and loss-of-function experiments both in vivo and in vitro.
3) Could asymmetric cell divisions, which generate two daughter cells that adopt different fates, be an intrinsic mechanism that helps generate cell diversity?
Previously, we have shown that some RPCs undergo asymmetric cell divisions and that the plane of the division regulate whether certain proteins will be symmetrically or asymmetrically inherited by the daughter cells. In particular, we showed that the protein Numb, a natural inhibitor of Notch signalling, is sometimes asymmetrically inherited by the daughter cells. However, the dynamics of the asymmetric distribution of Numb and the role of Numb in regulating specific cell fate decisions remain unclear. We are currently employing innovative live imaging technologies to track Numb protein distribution in living RPCs as they divide and differentiate to address these questions.